Installation of modem

Dear sir,
I am very new for linux operating system.I installed the linux server with help of some some document.But now i wants to connect a modem to my linux server & configure it for internet use.I also need help to how to share the single internet account and modem
for for LAN. I tried a lot of things but i was unsucesse to configure modem.Please give me step by step procedure & suggest me good help web site.
So please help me all of you

Thanks